The fixed, transparent mirror in the new Sony SLT series of interchangeable-lens cameras.
Sony Electronics

In what is--seriously--the worst-keptsecret of the summer, Sony officially announced the first members of its innovative new series of cameras, the Alpha SLT-A33 and SLT-A55V, which are the first cameras to incorporate translucent mirrors. I, along with about 15 other reviewers, got a chance to shoot with the cameras--as per our policy CNET footed the bill for my trip rather than Sony--and have sample photos and some preliminary analysis of the photo and video quality and ergonomics of the cameras.

Sony couples the translucent mirror with an updated 15-point autofocus system with three cross-type sensors (indicated in magenta).
Sony Electronics

Also known as a pellicle mirror, a TM passes most of the light from the lens through a fixed semitransparent mirror, reflecting a small bit of the light upward to a separate phase-detection autofocus sensor. This is how Sony achieves the faster phase-detect continuous AF for movie capture, while most current interchangeable-lens cameras (ILCs) and dSLRs use the slower contrast AF, which is based off the imaging sensor.

One doesn't necessarily need to use a mirror to incorporate phase detection, though; for example, Fujifilm's recent F300 EXR and F800 EXR point-and-shoots use a phase-detection array layered over the image sensor. Because many older dSLR-mount lenses can only work with phase-detection AF--that's why ILC adapters for older lenses generally don't support AF--Sony's system enables autofocus when using those lenses for shooting video.

Most traditional lenses have noisy focusing and aperture activation mechanisms, however, which makes them unsuited for video AF. Sony says it focuses so quickly that the noise should be barely perceptible, and that anyway you should use one of the new dedicated microphones (models ECM-ALST1 and ECM-CG50) for better separation from the lens. I didn't get a chance to test Sony's audio claims. The quieter lens is one of the rationales behind the electromagnetic activation in the NEX series' E-mount lenses, and it's hard not to feel like Sony's entering some confusing territory by pairing its most video-oriented still camera with its old-fashioned lens system. The cameras also update to a 1200-zone metering system.

The A33 has a traditional dSLR analog, the DSLR-A560, announced at the same time, which replaces the A550 and A500. Here's Sony's current sub-$1,000 Alpha lineup:

The SLTs seem to compete more with the dSLRs than they do with their ILC cousins, the NEX-3 and NEX-5 for a variety of reasons, including the shared lens mount, size, speed, and user interface. Unlike the NEX models, the SLTs have electronic viewfinders--probably the best I've ever used. But while I love EVFs for shooting video, and that's one of the things that give the SLT models a decided advantage over the A560, which otherwise has a similar set of video capabilities, there are still trade-offs between EVFs and optical viewfinders when it comes to burst shooting. Even the best EVF can't refresh quickly enough to allow for panning or easily following the subject. There are technological limitations that prevent the use of a traditional TTL optical viewfinder with this mirror design, though; while the mirror can bounce enough light for the phase-detection sensor, it's not sufficient to illuminate a viewfinder and you don't want to divert much more from the imaging sensor. A dynamic AF tracking mode would really be useful here, but I suspect Sony's saving that for an even more expensive model.

I give the cameras high marks for general photographic usability, and the drop-down articulated LCD, like the one on the Nikon D5000, comes in very handy. They function very much like Sony's standard dSLRs, which is a big plus over the NEX's paradoxically dumbed-down yet awkwardly arranged interface. Sony adds a new Auto+ mode, an automatic scene selection mode that the rest of the world already has, as well as its usual assortment of multishot modes like Auto HDR (increased to six shots for a possible 6EV increase in tonal range), Handheld Twilight, Sweep Panorama, and Sweep 3D. The geotagging on the A55 works seamlessly; as far as I could tell it didn't add any performance overhead and accurately tagged the photos. We'll see what happens when I put it to the test here in NYC, though.

The video capture, however, will probably annoy a lot of folks who are wishing it's more than it is, and it annoys me because the capabilities feel arbitrarily constrained--like Sony's planning to put them in a more expensive model. First, there are practically no manual exposure controls, and what few there are you can't set independently from still photos. There's an aperture-priority movie capture mode, but it only works with manual focus, and it locks the aperture wide open. This is likely to keep the A-mount lens' loud aperture activation from registering on the audio track.

But typical consumers used to point-and-shoot video, Flip camcorders, or even your typical sub-$600 HD camcorder will likely find it both comfortable to shoot video and will like the result (although as I mentioned I'm withholding judgment on the potential lens noise). Because of the fast AF, large sensor, and better metering system, the few pieces of footage I shot looked as good as anything I've shot with a decent consumer camcorder, and with some practice would probably look even better.

Ditto goes for the still photos. While they had a bit of the point-and-shoot look I don't expect from a camera in its price range, the A55 had very good noise reduction at ISO 6400 and 12800, and most consumers will find the continuous-shooting autofocus performance and photo quality good enough for shooting kids, pets, and sports--the mainstream dSLR trifecta. Keep in mind that a fast (30MB/sec) SD card will make a big difference in your burst performance experience.

The SLT-A33 and A55 are relatively expensive, especially for most people who are looking for dSLR capabilities at well less than $600. But their closest competitor is really the Panasonic DMC-GH1, which offers an even better set of video chops, albeit at a much higher price (in part due to the pricey lens it comes with). However, rumors abound that its successor is slated for announcement at Photokina in September, making that a wild card. Sony's translucent mirror technology is likely only one of a few solutions we'll see over the next year to solve the video AF problem in still cameras, but for the moment, it looks like it's a workable one for the mainstream--and one limited enough that won't cannibalize Sony's potential market for the NEX-VG10.